1. Field of the Invention
The subject invention relates to gas flow measurement.
2. Related Art
The accurate flow of gas is important in many industrial processes. In the semiconductor industry, transistor critical dimensions are ever shrinking to smaller and smaller technology nodes. As the critical dimension requirements become tighter, it is becoming increasingly important to improve the precision of the wafer processing to enable the fabrication of semiconductor chips at smaller nodes. For recipes that are dependent on an accurate flow of gases, the ability to measure the accurate flow rate of individual gases into the process chamber is extremely important.
Several solutions that allow for the measurement of gas flow into a process chamber have been developed as a response to the market need. However, most of the solutions do not allow for the accurate measurement of gas flows during chamber processing. Patent application Ser. Nos. 12/354,723 and 12/355,654 by Monkowski et al, describe a novel solution that allows for in-situ gas flow measurement of individual gases flowing into a process chamber. The solution describes the method of stopping the flow of gas upstream of a gas stick's flow control device (generally a mass flow controller) and measuring the rate of pressure drop in the volume between the location where flow has been stopped and the flow control device in order to calculate a flow rate.
The precision of prior art in-situ gas flow measurement is highly dependent upon the pressure reading from a pressure measurement device. Thus, prior art systems may provide accurate measurements in well-controlled environments. However, in less controlled environments, noise or other anomalies in the pressure measurement can adversely affect the gas flow calculation, decreasing its reliability and repeatability. Similarly, noise or an offset in the temperature reading can also affect the accuracy and repeatability of the gas flow calculation.
When taking pressure measurements, the accuracy of the timestamps associated with the samples can affect the gas flow measurement, since when calculating rate of pressure drop the accuracy of the sample timestamps are as important as the accuracy of the pressure readings. The issue is further complicated since it is commonly known that typical tool and chamber controller operating system software clocks have margins of error on the order of a few milliseconds, which could affect the gas flow measurement calculation significantly.
Additionally, non-linearities and discontinuities in the gas pressure readings due to set point changes or non-ideal gas compressibility factors may cause inaccuracies in gas flow measurements. Novel methods as described in the detailed description below can be employed to account for such non-linearities and discontinuities in order to provide more accurate gas flow measurement results.
From the above, it is seen that techniques for enhancing the robustness and performance of in-situ gas flow measurement systems are desired.